An Experiment to determine Water Potential in Potato Tissue.

Rikin Patel

An Experiment to determine Water Potential in Potato Tissue

Introduction

Osmosis is the movement of water molecules move from a region of high water concentration (high water potential) to a region of low water concentration (low water potential). The water molecules also move through a partially permeable membrane.

Osmosis affects the shape of plant cells. When a plant cell is in a lower water potential (highly concentrated solution), water from the plant cell moves out and therefore it becomes flaccid, and this is when the cell membrane of the plant cell shrinks. When a plant cell is in a solution which has higher water potential, the water moves into the cell and therefore it becomes turgid, this is when the cell membrane expands. In both cases, it is only the cell membrane that changes its shape. The cell wall always keeps its shape. The mass of plant cells change as well. This is because water is moved into or out of the plant cell and therefore a change occurs to its mass. At equilibrium (when the solution’s water potential and the plant cell’s water potential are the same) there will be no movement of water. Therefore the shape and the mass of plant cell would stay the same.

The tendency of water molecules to move from one place to another is measured as the water potential, which is represented by the sign Ψ. It is a Greek letter for Psi. The water molecules can move because they have kinetic energy and they can move randomly. Solute molecules (molecules which dissolve into liquid to form a solution) always lower the water potential. Pure water has the highest water potential, which is 0Ψ. All the other solutions have negative water potential. Therefore lower the water potential, the solution is more concentrated.

Water potential is affected by two main factors:

Solute potential
Pressure potential.

The effect of dissolving solute molecules in pure water causes it to reduce the concentration of water molecules and therefore lowers the water potential. This means that all solutions have lower water potential than pure water. The amount of this lowering is called solute potential. The solute potential is a measure of the change in water potential because of the presence of solute molecules.

The other factor that affects the water potential is pressure potential. As the pressure increases onto pure water or a solution, its water potential increases. This is caused by the kinetic energy, which tend to move the water molecule from one place to another. Overleaf is a diagram that illustrates osmosis.

- PPM

- Solute molecule

- Water molecule

Prediction

I think that as the concentration of sucrose solution increases, the mass of potato would decrease. Therefore in this experiment the water molecules would move from the solution into the potato for the weaker solutions (i.e. the solutions with molarity of 0.0 moldm-3, 0.2 moldm-3 and 0.4 moldm-3) yet for the other four solutions the water molecules will move from the potato into the solution. This will make the mass of the potato decrease.

My reason for saying this would happen is that water always moves from a region of high concentration to a region of low concentration and in the case of osmosis the molecules go through a partially permeable membrane. The membrane in the potato is the plasma membrane. When the water enters the potato cell it makes it rigid, this would be described as being turgid. When water leaves or diffuses out of the potato it makes the potato flaccid. If too much water leaves the cell it shrinks and becomes plasmolysed.

Variables

There are a number of variables in this experiment:

The concentration of the sucrose solution- to measure the different amounts of liquids I used a graduated pipette instead of a syringe, because it is much more accurate. This variable is a factor because the more concentrated the solution is then the amount of osmosis increases
Surface area of the potato- To measure the length of the potato I used a 15-cm rule. I could have used external callipers but they would only measure on bit of the potato, whereas with the ruler you would get an average. The surface area is a variable because the larger the surface area the amount of osmosis increases.
The mass of the potatoes- To measure the mass of the potato I used an electronic weighing machine. This is quite an important factor because the heavier the potato, the more cells it contains, thus more osmosis.
The length of time the potatoes are in the solution- the time that the potatoes were kept in the solution was the same. This is also relatively important because the longer the piece of potato stays in the solution then the amount of osmosis is increased
The temperature- if some of the solutions with the potato in them are put in the sunlight by accident, the rate of osmosis would be increased thus some of the boiling tubes would have more completely carried out osmosis.

Preliminary Work

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Preliminary Work

Preliminary work was done to get good ideas of what I should do in the real experiment. I tested three potatoes in 5 different concentration of sucrose solution. They were 0.0 moldm-3, 0.1 moldm-3, 0.2 moldm-3, 0.4 moldm-3, 0.8 moldm-3, and 1.0 moldm-3. All the potatoes were 3 cm long and the thickness of all the potatoes were kept same by using same sized cork borer. This was to make sure that they all had similar mass. I placed the potatoes into the solution. The total amount of solution was 20 cm3, the solution covered all the potato, and this was necessary for accurate results. Then I left the test tubes where the sunlight did not shine and left them for about 1 hour. After 1 hours, I took them out and dried the excess solution soaked into the potato by tissue. Then I measured them on an electronic balance; I recorded the results in 3 decimal places. Below are the results that I obtained from my first experiment:

Preliminary experiment showed many disadvantages and advantages of using the method above. Also it gave me ideas about what kind of figures I was going to achieve in the real experiment and what concentration I should test potatoes on.

Firstly I think using a normal pipette is not accurate enough therefore I’ll use granulated pipette. The result shows that water potential of potato is some where between 0.1 moldm-3 concentration and 0.2 moldm-3 concentrations. Therefore when I do the real experiment I wouldn’t bother doing the higher concentration. I will be testing concentrations from 0 to 1.0 moldm-3 every 0.2 moldm-3.

Also I will make the length of potatoes 25 mm rather than 30 mm because if it is smaller there is less chance of potatoes being not covered by the solution.

Method

The purpose of this experiment was to find out the water potential of different samples of potato tissue. The potato was put into different concentrations of sucrose solution. In this experiment we used a number if apparatus. These included:

Boiling tubes
Boiling tubes rack
2 × 250 cm3 beakers
Cork borer
Scalpel
Ceramic cutting tile
Electronic balance
10 cm3 Pipette
Pipette filler
15 cm rule
Potato
Solution (including water and sucrose)

For each different concentration of solution I used three test tubes, this was so I could find an average for the solution. This required me to use eighteen test tubes; each of the eighteen test tubes had a piece of potato. I used the cork borer to remove the samples of potato. These eighteen samples of potato were all cut precisely 25 mm in length. All the samples of potato were cut using the same cork borer thus they all had the same circumference. This means if the circumference is the same and the length is the same, then the surface area of the potato will also be the same. The concentration of the sucrose solution ranged from 0.0 moldm-3 to 1.0 moldm-3. All together there were six different concentrations of sucrose solution, 0.0 moldm-3, 0.2 moldm-3, 0.4 moldm-3, 0.6 moldm-3, 0.8 moldm-3 and 1.0 moldm-3. I think that the change from the first to the last variable were suitable because they go across the span in the most even way possible. The total amount of solution in each boiling tube was 20 cm3. There were different amounts of water and sucrose solution in each of the 6 solutions, depending on the concentration of the final solution. Below is a table of all the different concentrations of solution:

In this experiment the only variable that I change was the different concentrations. The variables that were all kept the same, were the length of the potato, the circumference of the potato tissue and the period of time that I kept the potato in the solution, which was approximately 20 hours. To make sure that this was a fair experiment I used the same sources of distilled water and sucrose solution, this would insure that if there were any chemical present in any of the two solutions then all my boiling tubes would contain them. I also used the same size of cork borer, this because a different cork would increase or decrease the surface area of the potato thus a faster or slower rate of osmosis. The boiling tubes were all the same size. I also always used the same electronic balance. This was just a precaution in case there was an electronic error (i.e. if the reading shown was more or less than the actual mass.).

Firstly I used the cork borers to get the eighteen samples of potato tissue. I then used the scalpel to cut the potato to the required length, which was 25 mm. I used a ceramic tile to ensure there is no damage to the work surface. I put these pieces of potato on a piece of tissue and left them on the side. I then mixed all the solutions, putting the required amounts of sucrose and water. I put all the solutions in individual boiling tubes. Before I put the pieces of potato into the solution I measured the masses of every of the potato. I then put one piece of potato into a test tube, the potatoes stayed in the solution for approximately 20 hours. This is a satisfactory length of time for osmosis to take place, so that a sufficient change in mass takes place. After the 20 hours I removed all the pieces potato from the solution at the same time so it remained a fair test. I then the tissue paper to dry the potato lightly. This is because I had to measure the mass of the potato again and the solution that is left on the potato would increase the mass, thus being an unfair test. I then calculated the change in mass of the potato. The first measurement I took was the mass of the potato before it was put into the solution. After the potato tissue had been in the solution for the required amount of time I measured the mass again. I then subtracted the final mass from the initial mass, to find out the change in mass. All the masses of the potato were measured in grams, to 3 decimal places. After weighing the mass of all eighteen samples of potato, I had to know find out the average percentage change in the mass. To do this I had to firstly find out he change in mass, then I used following equation:

Percentage Change in Mass = Change in mass × 100

Initial Mass

Something that I observed in the experiment was that when I removed some potato samples from the solutions that contained high amounts of sucrose thus they were flaccid. This is because the water from in the potato cells has diffused out of the cell through the plasma membrane and into the solution. This process is called osmosis. The water makes the potato turgid. So as the water has diffuses out, through osmosis the cells lose their turgidity.

The main aim of this experiment is to determine the water potential of the potato tissue, therefore to help me find out the water potential in the potato tissue I plotted a graph that showed the solute potential of given sucrose solutions. The graph has Molarity against solute potential (Kpa). To calculate the water potential I would use the following equation:

Water Potential = Solute Potential + Pressure Potential

Solutes make water potential lower, therefore the water potential drops below zero. This means that the more solute in a solution, then the water potential will be lower. Solute potential is the amount the solute molecule is lower then the water potential. Pressure potential is the amount of pressure that is applied on the plasma membrane by water diffusing into the cell, when the cell is in a solution of higher water potential.

Safety Precautions

This experiment does not have a large number of safety precautions. The basic precaution that should be maintained this experiment is the use of safety goggles. This is due to the fact that this experiment is dealing with liquids and an certain individual may have a allergic reaction if any of the liquids enter the eye. The second safety point that is also very important is the use of the scalpels. They should always be used in a downwards motion, i.e. the blade of the scalpel should always move towards the ground. The scalpel should also be used on a tile. Another safety point is to do with the boiling tubes, whenever you are using a boiling tube you should always point the test tube away from an individual. This is because if the liquid in the tube start to boil and then starts spitting, then it will not cause anyone any injury.

Results

The average change in grams of Potato Tissue for each Molarity

Conclusion

The mass of potato decreases as the concentration of sucrose solution increases. As you can see on the graph the rate goes down as the concentration goes up. The prediction matched the results. The graph shows that the rate goes down a lot steadily then it flattens out to level up however not completely horizontal. The rate was quite steady from 0.0 moldm-3 until concentration was 0.6 moldm-3. From 0.4 moldm-3 of concentration to 1.0 moldm-3, the rate was slowed down. The gradient of the first half of the graph was steeper than the second half.

All the results showed change in the mass of potato and therefore proves that osmosis took place. When concentration of sucrose solutions were 0.0, 0.2 and 0.4 moldm-3, the mass of the potato cells increased. This is because the concentration 0.0, 0.2 and 0.4 moldm-3 of sucrose solution had higher water potential than the potato cells. As I already mentioned in introduction, osmosis is movement of water from high water potential from to lower water potential through partially permeable membrane. When potatoes were placed in those concentrations for 20 hours, the water molecules from the solution moved into the potato cells and caused to increase the mass and also made the cell ‘turgid’ in which the membrane stretched. The water potential is affected by two things, which are Solute potential and pressure potential. Here solute potential affected the mass to rise. It also decreased the change in mass as the concentration increased because solute potential decreased as amount of solute molecules increased. As I mentioned in prediction this creates decrease in mass of potato cells because water potential gets lower as well.

From 0.6 to 1.0 moldm-3, the mass decreased. This is because osmosis made the water from the potato cells to move out to the solution. This proves that water potential of sucrose solution from 0.6 to 1.0 moldm-3 were lower than the water potential in potato cells. This would have made the cells to be come ‘flaccid’ in which the cell membrane shrunk and also lost its mass due to more water moving out. Losing water makes large difference to its weight because plant cells are mainly made out of water molecules.

Another thing I should mention is that from 0.6 to 1.0 moldm-3 of concentration, the rate of decrease flattened. I think that this happened because it got to a point where there wasn’t enough water left in the potato cells to diffuse out. I can support this because the size of the potato that was left in concentration 1.0 moldm-3 after 20 hours was much smaller than one that was left in 0.0 moldm-3 sucrose solutions. Also the colours of potato from 1.0 moldm-3 solutions were slightly darker than the one from 0.0 moldm-3 solutions, which was quite pale. This shows that there was more water left in potato from 0.0 moldm-3 solution.

From the conclusion above it proved that from 0.0 to 0.6 moldm-3, the mass went up and from 0.6 to 1.0 moldm-3, the mass of potato went down. This indicates that the water potential of potato cell is some where between 0.4 and 0.6 moldm-3. Reading off my line of best fit, it shows that the water potential of potato is same as 0.21 moldm-3 of sucrose solution.

When the percentage change in mass on my graph is zero the pressure potential is also is also zero, at this point the solute potential is –540 Kpa. This means the pressure potential is also zero because there has been no change in the mass seeing as no water has diffused in or out. Therefore as solute potential is the same as water potential when the pressure potential is zero the water potential also equals –540 Kpa.

Evaluation

As I mentioned in conclusion, the actual water potential of potato cell and my result was very different. I think this is caused by experimental errors.

The technique of weight measuring might have caused many problems. I might not have settled the weight measurer completely before I used it. Also some water or solution might have been left on weight measurer from person before which could have affected the mass. To improve this problem, make sure that you dry the plate before you place your potato and also let the weight measurer settle completely before using it.

I think that when I was cutting up the pieces of potatoes, the accuracy of measuring length was very poor. I only had ruler to measure it with and therefore the mass of the potatoes was not as close as I wanted them to be. This can be easily improved by using more accurate equipment. For example, I could use magnifying glass so that I can cut the potato to the nearest accuracy.

Using tissue caused many problems. This is because there are no accurate way of finding out how much excess water is absorbed into tissue. This would have made the results less reliable because water can change the mass a lot. To achieve best accuracy out of this method I made sure that I always used same type of tissue. Also I made sure that the tissue was not wet before drying.

Using measuring cylinder was also a problem. Getting the right ratio of distilled water and sucrose solution was very difficult. This is because measuring cylinder scales were very narrow and getting the right ratio was hard. Also if you put in too much or too small, it was hard to sort out as you don’t know whether to add more distilled water or sucrose solution.

I think that the biggest problem was drying the soaked potato pieces before weighing. The problem is that I might have dried one potato piece much more than the another piece. This defeats the whole object of this experiment in which you are meant to find out the mass after water molecules have been moved around. To insure that this wouldn’t be a problem, I think tissue shouldn’t be used to dry however appropriate drying equipment should be used.

I think my results were pretty reliable. My repeated results were very similar for nearly all of them. They are all very close e.g. on concentration 0.4 moldm-3; the max difference was 0.04 grams, which is very little. Also on concentration 0.2 moldm-3, maximum difference was 0.05 grams, which is even more accurate. This proves that my results were very reliable.

To improve the reliability, I would do more repeats. This will ensure that reliability of results are very high as if there is a faulty result, it would stand out clearly if there are more repeats.

Also doing more concentrations and narrowing the scale of concentration would improve reliability. (i.e. doing 0.1, 0.15, 0.2, 0.25, etc).

Also using more accurate measuring equipment would have increased reliability, especially for measuring length.

I could have used different methods. Instead of measuring the change in mass, I could have measured the volume. Therefore in this experiment, if the volume stays the same, that would be the water potential of potato. I think in some ways this could be more accurate because no tissue is needed which caused most problems for my experiment when I was measuring the mass. However, measuring the volume would be very difficult.

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Teacher Reviews

Here's what a teacher thought of this essay

Stevie Fleming

29th of May 2013

**** Overall a sound experimental report that demonstrates a thorough experimental approach to calculating the water potential of potato tuber cells. To improve: Planning: The candidate produced a testable hypothesis backed up some relevant biological theory although there were some inaccuracies in the explanations. It would help to include more selected background reference material to help inform the plan. This should be referenced. The readability could be improved if some of the key information was in the form of tables rather than long paragraphs of text. Carrying out: A trial experiment was used to help inform the plan and a good range of the independent variable was selected. Tables could be improved by ensuring full details are included in the headings. Analysis and Evaluation: The graph was discussed but was not included. The discussion of the results was thorough and was explained using relevant biological theory. The writer tried to explain anomalous results. The evaluation could have been improved by making the suggestions for improving the method or range of the work more specific. For example the suggestion to use more accurate equipment should have included the instruments to use for this task.

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